This invention relates generally to the field of turbochargers and, more particularly, to a turbine shaft thrust bearing having separate oil inlet and oil outlet paths across a bearing axial surface, thereby providing improved bearing thrust load capacity and reduced oil film temperature.
Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing. The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft. Thus, rotary action of the turbine also causes the air compressor to spin within a compressor housing of the turbocharger that is separate from the exhaust housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
The common shaft extending between the turbine and compressor is disposed through a turbocharger center housing that includes a bearing assembly for: (1) facilitating shaft rotation; (2) controlling axially directed shaft thrust effects and radially directed shaft vibrations; (3) providing necessary lubrication to the rotating shaft to minimize friction effects and related wear; and (4) providing a seal between the lubricated assembly and the turbine and compressor housings. The common shaft as used in turbocharger applications is known to have shaft-rotating speeds on the order of 60,000 to 80,000 rpm, and in some applications up to 280,000 rpm. Under such operating conditions it is imperative that the bearing assembly provide sufficient lubrication to the shaft to minimize the extreme friction effects that take place at such high rotating speeds, thereby extending shaft service life.
A thrust bearing is installed in the turbocharger center housing and is generally used to support the axially directed thrust, or thrust load, that is imposed on the turbine shaft. The thrust bearing can either be hydrodynamic or non-hydrodynamic. As used herein, the term xe2x80x9chydrodynamicxe2x80x9d is understood to refer to pumped oil migration, or diverted oil flow, across an axial face of a thrust bearing, and the term xe2x80x9cnon-hydrodynamicxe2x80x9d is understood to refer to a type of thrust bearing that is not designed to pump oil across an axial thrust bearing face or surface.
Hydrodynamic thrust bearings known in the art comprise an annular body that is disposed within the turbocharger bearing housing, around the turbine shaft. Such bearing includes one or more axially-facing load-bearing surfaces. Turbocharger thrust bearings are usually two-sided, and are required to react loads in both axial directions. This concept applies whether one two-sided bearing handles thrust in both directions, or whether the thrust directions are reacted through two separate bearings. The load-bearing surfaces are is positioned adjacent an opposed thrust runner surface that is attached to and rotates with the shaft assembly. The load-bearing surfaces include a number of grooves or channels that are disposed radially thereacross from an inside bearing diameter to an outside bearing diameter. Each such groove is configured identically and is separated by a surface section or pad.
In such conventional hydrodynamic thrust bearing the load-bearing surface, comprising the arrangement of grooves and pads, is designed to distribute lubricating oil thereover in the following manner. Fresh oil is pumped into an inlet end of a oil groove that is positioned at a leading inside diameter edge of each load-bearing pad section. The oil migrates radially outwardly in the groove, circularly over the pad and radially towards a load-bearing surface outside diameter. As the oil is moved over the pad it is heated by the thrust bearing surface and the shearing action of the viscous load-carrying film. The oil continues to move circularly to a trailing edge of the load-bearing pad where it is passed to an adjacent radial groove that is operating to provide supply oil to an adjacent/downstream pad. Thus, the hot oil exiting one pad exits the load-bearing surface via a groove that is also used to provide cool supply oil to an adjacent pad section, thereby adversely impacting the ability of such thrust bearing design to provide a desired degree of cooled oil to the load-bearing surface sections.
Hydrodynamic thrust bearings provide a thrust load capacity that is dependent on the operating temperature of the oil film disposed between the thrust bearing and the adjacent rotating thrust runner. It is a well known fact that the thrust load capacity for such bearings is inversely proportional to the oil film temperature across the bearing. It has been discovered that the above-described hydrodynamic thrust bearings do not provide a maximum degree of thrust load capacity because of the high oil-film temperatures that are experienced across the bearing. A reduced thrust load capacity has an adverse impact on turbocharger service life as is allows undesired turbine shaft axial play that causes premature turbine bearing and seal wear. An inefficient thrust bearing design also leads to higher frictional losses in the turbocharger""s bearing system, and lower overall turbocharger mechanical efficiency.
It is, therefore, desirable that a hydrodynamic thrust bearing be constructed that has an improved thrust load capacity when compared to conventional hydrodynamic thrust bearings. It is also desired that such thrust bearing be capable of increasing the thrust load capacity without adversely impacting other performance areas of the thrust bearing itself and the bearing assembly. It is further desired that such thrust bearing be capable of fitment with existing turbocharger devices without extensive redesigning.
Hydrodynamic thrust bearings, constructed according to principles of this invention, comprise an axially-directed annular surface that is specially designed to promote oil migration thereacross in a manner that minimizes oil film temperature, thus maximizing thrust load capacity. Specifically, hydrodynamic thrust bearings of this invention comprise a body having a load-bearing surface that includes an oil supply channel and an oil return channel disposed along respective inside and outside body diameters.
The thrust bearings body includes a number of thrust pads positioned between the oil supply and return channels. An oil supply groove is positioned along each thrust pad leading/upstream edge and extends radially a distance from the oil supply channel. An oil return groove is positioned along each thrust pad trailing/downstream edge and extends radially a distance from the oil return channel. The oil supply and oil return grooves are separate from one another.
In a preferred embodiment, the thrust pads comprise a series arrangement of three differently configured land sections. Configured in this manner, oil is provided onto the series of land sections by the oil supply groove and is collected after passing over the series of land sections by the separate oil collection groove. The use of separate oil supply and collection grooves operates to minimize undesired oil mixing, i.e., supply oil mixing with the heated or discharge oil, thereby reducing oil film temperature and increasing bearing thrust load capacity.